1. Field of the Invention
The present invention relates to a vacuum pressure forming apparatus and vacuum pressure forming method which are available for manufacturing simple containers such as yogurt containers from a thermoplastic resin sheet.
2. Description of the Prior Art
In the so-called known vacuum (pressure) forming techniques, thermoplastic resin sheet member, after being heated by an appropriate heating device and softened, is fixed onto a surface of a metal pattern having a number of small vacuumizing holes so as to brought into a hermetically sealed condition so that no leakage of air occurs between the sheet member and the metal pattern. In this state the air is rapidly discharged from the cavities of the metal pattern through the small vacuumizing holes, with the result that the sheet member is formed into a configuration of the cavities due to the inversion transfer. Accordingly, for producing high-quality formed pieces through the use of such a forming technique, an important point is that the sheet member is previously heated to a uniform temperature. Otherwise, an irregular heating temperature distribution immediately causes production of articles of inferior quality, which include defects such as non-uniformities in wall thickness.
So far, for such a vacuum pressure forming apparatus, an oven type apparatus and a heating plate type apparatus have been known as a means for heating the sheet member. Although in the oven type apparatus both side portions of a formation sheet member in its width direction are clamped and heated by passing through heated air within an oven or the like, and thus the heated sheet member is softened and stretched as the temperature rises, so that its central portion hangs down due to its dead weight. If the sheet is hanging down during the formation by the metal pattern, there is a possibility that wrinkles will appear in the formed pieces.
In addition, in general the oven is constructed to be less than 1 meter in width by less than several meters in length, and in a case of heating by the oven, several tens to more than one hundred casing heaters made from ceramic or metal are vertically separately disposed at a considerable distance (10 to 20 cm) in a state that the sheet member to be formed is interposed therebetween. Each of the heaters is individually made such that its temperature is adjustable. Moreover, the entire heating is proportionally adjustable, and the total capacity of the heaters can be several tens of Kw to above 100 Kw. This is a capacity to the extent that the sheet member to be formed (formation sheet member) is fully carbonized when the total energy is absorbed by the formation sheet member, and from this point of view this apparatus is extremely uneconomical. Besides, because the temperature control is based upon the indirect heating (radiation heat and air heat transfer), adjustment of the temperature is necessary while watching out for the sheet member and the formed pieces, which in turn requires experienced and skilled workmen.
On the other hand, the heating plate type apparatus can provide high thermal efficiency, but is constructed such that a large number of heating elements are placed below the heating plate and controlled individually as in the case of the above-mentioned oven type apparatus. In addition, the heating plate type apparatus works as shown in FIGS. 3A to 3D. That is, after a sheet member 1 is placed on a top surface of a heating plate 2 (see FIG. 3A), a metal pattern 3 is lowered to tightly hold an outer circumferential edge portion of the sheet member 1 in cooperation with the heating plate 2. Subsequently, pressurized air is introduced through small holes 3a made in the metal pattern 3 into the interior of the metal pattern 3 so that the pressurized air presses the sheet member 1 to make it come closely into contact with the surface of the heating plate 2. In this state, the heating plate 2 heats the sheet member 1 (see FIG. 3B). Following this , the small holes 3a are released to the atmosphere (or vacuumed) and pressurized air is introduced through a number of air holes 2a formed in the heating plate 2 so that the sheet member 1 is pressed against cavity surfaces of the metal pattern 3 (see FIG. 3C). Finally, the metal pattern 3 is lifted to remove the pieces formed therein (see FIG. 3D).
In the case of such a heating plate type apparatus, when the sheet member 1 is pressed by air pressure against the surface of the heating plate 2, as shown in FIG. 3B, an air layer stays between the sheet member 1 and the surface of the heating plate 2 so that portion of the sheet member 1 does not come closely into contact with the heating plate 2 surface to cause unsatisfactory heating, which can bring about rain drops.
One possible approach to reduce the occurrence of the rain drops is to Teflon-process or roughly machine the surface of the heating plate in order to facilitate the air discharge and the movement of the sheet member when heated or the temperature of the heating plate is adjusted relative to the percentage of heat contraction of the sheet member.
However, in general the heated sheet member lengthens due to the thermal expansion so as to enlarge its surface area. Also, since in this type of heating plate apparatus, the outer circumferential edge portion of the sheet member is hermetically held between the heating plate and the metal pattern and pressurized air is then introduced into the metal pattern to heat the sheet member. In other words, since in the heating plate type apparatus the sheet member undergoes area enlargement after the fixing, of its circumference, the expansion of the sheet member accordingly produces looseness so that a portion of the sheet member naturally rises to some extent, which destroys the close contact between the sheet member and the heating plate, thereby making it difficult to uniformly heat the sheet member. This problem is unavoidable because of the principle of the heating plate type apparatus in which the circumference of the sheet member is clamped prior to the heating operation.